instead using a green screen/colour key, a luma-key?
Yep, you could replace that in Motion… but luma-keys are never as good as colour-keys, in my example, the guitar player would get partly transparent…

attached a quick'n dirty variation of my plugin. Again:
Motion does not allow to publish the GUI of the keyer, therefor I published a few, hopefully useful parameters

Thanks. It doesn't work though: info tells me the effect is missing. Installed it at the same location as the original Feedback effect, haven't changed the name or anything. Showing the dreaded red screen with yellow triangle exclamation mark. Eeks!

Thanks a lot for you ultra-quick replies and fixes.
Unfortunately I haven't upgraded FCPX to latest version yet (mid-project, I'll go 10.3 in December).
The hacked version doesn't work either. I guess I'll manually put a couple of copied layers on top of each other.

This would demand a very different concept to construct such a plugin, esp. not using a Replicator
Quick shot from the hip: the Particle Behavior allows to 'Shift' the playback start…
Actually busy with other things… maybe in the near future I can tinker with such a feature…